<HTML>
<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A> 
</CENTER>






<HR>

<H3>dihedral_style class2 command 
</H3>
<H3>dihedral_style class2/omp command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>dihedral_style class2 
</PRE>
<P><B>Examples:</B>
</P>
<PRE>dihedral_style class2
dihedral_coeff 1 100 75 100 70 80 60
dihedral_coeff * mbt 3.5945 0.1704 -0.5490 1.5228 
dihedral_coeff * ebt 0.3417 0.3264 -0.9036 0.1368 0.0 -0.8080 1.0119 1.1010
dihedral_coeff 2 at 0.0 -0.1850 -0.7963 -2.0220 0.0 -0.3991 110.2453 105.1270 
dihedral_coeff * aat -13.5271 110.2453 105.1270 
dihedral_coeff * bb13 0.0 1.0119 1.1010 
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>class2</I> dihedral style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/dihedral_class2.jpg">
</CENTER>
<P>where Ed is the dihedral term, Embt is a middle-bond-torsion term,
Eebt is an end-bond-torsion term, Eat is an angle-torsion term, Eaat
is an angle-angle-torsion term, and Ebb13 is a bond-bond-13 term.
</P>
<P>Theta1 and theta2 are equilibrium angles and r1 r2 r3 are equilibrium
bond lengths.
</P>
<P>See <A HREF = "#Sun">(Sun)</A> for a description of the COMPASS class2 force field.
</P>
<P>Coefficients for the Ed, Embt, Eebt, Eat, Eaat, and Ebb13 formulas
must be defined for each dihedral type via the
<A HREF = "dihedral_coeff.html">dihedral_coeff</A> command as in the example above,
or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands.
</P>
<P>These are the 6 coefficients for the Ed formula:
</P>
<UL><LI>K1 (energy)
<LI>phi1 (degrees)
<LI>K2 (energy)
<LI>phi2 (degrees)
<LI>K3 (energy)
<LI>phi3 (degrees) 
</UL>
<P>For the Embt formula, each line in a
<A HREF = "dihedral_coeff.html">dihedral_coeff</A> command in the input script lists
5 coefficients, the first of which is "mbt" to indicate they are
MiddleBondTorsion coefficients.  In a data file, these coefficients
should be listed under a "MiddleBondTorsion Coeffs" heading and you
must leave out the "mbt", i.e. only list 4 coefficients after the
dihedral type.
</P>
<UL><LI>mbt
<LI>A1 (energy/distance)
<LI>A2 (energy/distance)
<LI>A3 (energy/distance)
<LI>r2 (distance) 
</UL>
<P>For the Eebt formula, each line in a
<A HREF = "dihedral_coeff.html">dihedral_coeff</A> command in the input script lists
9 coefficients, the first of which is "ebt" to indicate they are
EndBondTorsion coefficients.  In a data file, these coefficients
should be listed under a "EndBondTorsion Coeffs" heading and you must
leave out the "ebt", i.e. only list 8 coefficients after the dihedral
type.
</P>
<UL><LI>ebt
<LI>B1 (energy/distance)
<LI>B2 (energy/distance)
<LI>B3 (energy/distance)
<LI>C1 (energy/distance)
<LI>C2 (energy/distance)
<LI>C3 (energy/distance)
<LI>r1 (distance)
<LI>r3 (distance) 
</UL>
<P>For the Eat formula, each line in a
<A HREF = "dihedral_coeff.html">dihedral_coeff</A> command in the input script lists
9 coefficients, the first of which is "at" to indicate they are
AngleTorsion coefficients.  In a data file, these coefficients should
be listed under a "AngleTorsion Coeffs" heading and you must leave out
the "at", i.e. only list 8 coefficients after the dihedral type.
</P>
<UL><LI>at
<LI>D1 (energy/radian)
<LI>D2 (energy/radian)
<LI>D3 (energy/radian)
<LI>E1 (energy/radian)
<LI>E2 (energy/radian)
<LI>E3 (energy/radian)
<LI>theta1 (degrees)
<LI>theta2 (degrees) 
</UL>
<P>Theta1 and theta2 are specified in degrees, but LAMMPS converts them
to radians internally; hence the units of D and E are in
energy/radian.
</P>
<P>For the Eaat formula, each line in a
<A HREF = "dihedral_coeff.html">dihedral_coeff</A> command in the input script lists
4 coefficients, the first of which is "aat" to indicate they are
AngleAngleTorsion coefficients.  In a data file, these coefficients
should be listed under a "AngleAngleTorsion Coeffs" heading and you
must leave out the "aat", i.e. only list 3 coefficients after the
dihedral type.
</P>
<UL><LI>aat
<LI>M (energy/radian^2)
<LI>theta1 (degrees)
<LI>theta2 (degrees) 
</UL>
<P>Theta1 and theta2 are specified in degrees, but LAMMPS converts them
to radians internally; hence the units of M are in energy/radian^2.
</P>
<P>For the Ebb13 formula, each line in a
<A HREF = "dihedral_coeff.html">dihedral_coeff</A> command in the input script lists
4 coefficients, the first of which is "bb13" to indicate they are
BondBond13 coefficients.  In a data file, these coefficients should be
listed under a "BondBond13 Coeffs" heading and you must leave out the
"bb13", i.e. only list 3 coefficients after the dihedral type.
</P>
<UL><LI>bb13
<LI>N (energy/distance^2)
<LI>r1 (distance)
<LI>r3 (distance) 
</UL>
<HR>

<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
the same as the corresponding style without the suffix.  They have
been optimized to run faster, depending on your available hardware, as
discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
manual.  The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.
</P>
<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
packages, respectively.  They are only enabled if LAMMPS was built with
those packages.  See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
section for more info.
</P>
<P>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
switch</A> when you invoke LAMMPS, or you can
use the <A HREF = "suffix.html">suffix</A> command in your input script.
</P>
<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
more instructions on how to use the accelerated styles effectively.
</P>
<HR>

<P><B>Restrictions:</B>
</P>
<P>This dihedral style can only be used if LAMMPS was built with the
CLASS2 package.  See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
section for more info on packages.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "dihedral_coeff.html">dihedral_coeff</A>
</P>
<P><B>Default:</B> none
</P>
<HR>

<A NAME = "Sun"></A>

<P><B>(Sun)</B> Sun, J Phys Chem B 102, 7338-7364 (1998).
</P>
</HTML>
